From the files of the What Can't Dropbox Handle Dept.: the web-based file syncing service makes it easy to keep custom dictionaries in Word, or most any other app, available and synchronized between computers, as commenter caryo points out.

From the "Spelling and Grammar" tab in Microsoft Word's options, you can choose where your custom dictionary—with all your custom words for the spell checker not to catch—should reside, and export it somewhere else. Move it into your Dropbox folder, and set up other computers to pull their custom dictionaries from there, too. When you add a word into one system, it gets updated on others. Browsers and other apps also keep custom spelling dictionaries (here's where Firefox keeps its own), so Dropbox, and other instant file-syncing services, can be very handy indeed.